1. Field of the Invention
The subject invention relates to a circuit for modifying a color video signal, and more particularly, to a circuit for assuring that the front porch portion of a color video signal is at an appropriate signal level to allow for proper decoding of the video signal.
2. Description of Related Art
In color television, horizontal synchronization and color information is coded and included in the video signal. In order for a decoder in the television receiver to properly decode this color information, it is necessary for the decoder circuitry within the television receiver to properly synchronize itself with the transmitted color video signal.
FIG. 1a shows the video signal for a single horizontal scanning line of a typical color video signal. This scanning line includes a front porch area 10 followed by the horizontal synchronizing pulse 12. The pulse 12 is then followed by the color burst 14, the back porch area 16 and the video information 18 for the line. The color burst 14 contains the necessary information enabling the decoder to decode the color video signal. Hence, as indicated above, in order for the receiver to properly decode the signal, it is essential for the decoder circuitry to accurately detect the position of the horizontal pulse.
FIG. 1b shows a video signal similar to that shown in FIG. 1a with the exception that the front porch area 10 is sagging, that is, at a much lower level than in FIG. 1a. This situation may occur in the playing back of certain video tapes on a video cassette recorder. While prior art analog television receivers could adequately compensate for this lowered front porch, present day television receivers using digital circuitry in the decoder encounter problems with this sagging front porch signal which results in the digital synchronization processor in the decoder not being able to distinguish the actual beginning of the horizontal synchronizing pulse causing a large saturation change and a loss of horizontal synchronization. This appears as either severe horizontal tearing or slanted noisy black bars on the display screen of the television receiver.